Debt (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Short-term debt outstanding	$ 373,144,000	$ 268,398,000
Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt outstanding	373,144,000	268,398,000
Weighted average interest rates on short-term debt outstanding (in hundredths)	0.20%	0.30%
Average daily borrowings	$ 436,700,000	$ 282,000,000
Weighted average interest rates on average daily borrowings (in hundredths)	0.20%	0.30%